Fresh snow and erratic winds have created wind slabs on all aspects above 4,500' feet On December 26th, we found these slabs to be reactive to human triggering on steep convex rolls and on slopes below saddles where the higher winds were funneling and depositing fresh snow onto leeward aspects. These slabs were not highly reactive and were still quite soft. We noticed that they increased in both cohesiveness and reactivity as the day progressed due to a steady pummeling from the wind. These thin, soft slabs will likely become more reactive and dangerous with the predicted weather moving in on Tuesday, December 27.
Backcountry travelers should exercise caution when traveling through areas that are showing signs of recent wind loading. Look for soft pillow like appearance in the snow. Signs of cracking and density changes in the snow should be taken as obvious clues that you may need to exercise extra caution and stick to low angle terrain that is protected from the wind. Avoid steep convex rolls as these slabs are proving to be very reactive on this type of terrain feature.
The mountains of the Kootenai National Forest are forecasted to recieve close to a foot of snow and strong westerly winds in the next 24 hours. This scenario will likely create a substantial rise in the potential for both natural and human triggered avalanches. Backcountry travelers can mitigate this hazard by sticking to low angle terrain (less than 30 degrees) until this new snow has had ample time to strengthen and settle.
The wind slabs of the previous two weeks have become less reactive with time and are now becoming buried within the snowpack. We are not finding them to be reactive to human triggers or snow stability tests; however, they are worth paying attention to as additional loading is added to the snowpack. These higher density layers are overlying a wide variety of weaknesses throughout the mountians of the Kootenai Region. On northerly aspects we are finding buried surface hoar and faceting, depth hoar development in areas with a shallow snowpack, and old rain crusts lying underneath a layer of sugary facets at elevations above 6,500'.
As I mentioned these weaknesses are not proving to be reactive at this time. This could change with addtional loading, a rapid rise in temperature, or a human trigger launching off a cornice on skis or a snowmobile. This problem can be managed by assessing each slope individually before committing to it and practicing safe travel habits.
On December 26, we traveled into the East Cabinets above the Snowshoe Lakes. The most obvious avalanche hazard we found consisted of the new snow (4-6 inches) that had been transported by wind. We found the wind to be battling in direction through out the day which was causing wind slab development on all aspects. The slabs became more reactive as the day progressed and we were able to trigger some soft slabs on steep convex rolls that were soft and 8-10" thick. Our stability tests and pit results gave us a wide variety of results with very subtle changes in aspect and elevation, constant change is the only consistency so far this year. None of the weaknesses in the snowpack propagated in the extended column test or proved to be very reactive at this time. There are numerous weak layers to monitor as the season progresses and I would encourage backcountry travelers to dig into the snow and see whats going on underneath your skis or snowmobile.
The next Snowpack Summary wil be posted Saturday afternoon on December 31st.
On December 23, we traveled to Flatiron Mountian in the Purcell Range where we found a very thin and sugary snowpack with extensive faceting, depth hoar development, and buried surface hoar. It was the begining of a very poor base for the Purcell snowpack and will be something to watch as winter progresses and more cohesive layers are added over time.
For folks riding in the West Cabinets and areas near the Idaho-Montana border check out the forecast from the Idaho Panhandle @ http://www.idahopanhandleavalanche.org/current-advisory.html
You can help us get info out as well by posting your observation by clicking on the links to the left and tell us what you are seeing out there.
Backcountry Forecast from NWS Missoula issued:
340 AM MST Tue Dec 27 2016
DISCUSSION: Snow and wind started to move into western mountains
overnight and this morning, and is expected to increase in
intensity through the day. Westerly winds gusting up to 50+ mph is
likely for the ridge tops today. We'll see a brief break in the
active weather Thursday, before another weak disturbance moves
through Friday, with another round of several inches of snow. A
much stronger system is expected toward the end of the weekend and
early next week.
--------------------------- 5000-7000 FT ----------------------------
Today Tonight Wed
Cloud Cover 95% 85% 75%
Hi/Lo Temps 22 to 26 14 to 20 20 to 24
Winds(mph) SW 19G37 W 18G39 SW 13G28
Precip Chc 100 60 30
Precip Type snow snow sno/shr
Liquid Amt 0.36 0.08 0.02
Snow Ratio(SLR) 18:1 18:1 18:1
Snow Amt(in) 5-11 1-3 0-1
Snow Level 0 500 0
This advisory applies only to backcountry areas outside established ski area boundaries. This advisory describes general avalanche conditions and local variations always occur. This advisory expires midnight on the posted day unless otherwise noted. The information in this advisory is provided by the USDA Forest Service who is solely responsible for its content.